How can I make any size object (matchbox,car) fit within the camera view?

every model I feed the runtime
to assume the camera’s viewing
boundaries, so it’s always the same
fit whether it’s a car, a building or
a spec of sand

but then

I am actually trying to replicate the
exact function of F button in viewport

I thought you wanted to blow up / scale down a model to fit into a static camera frustum accounting for fov, aspect ratio and so on. If you zoom in on a spec of sand, you’re gonna have bad time if you just move the camera around.

Other than that, ClockworkOcean seems to have you covered.